User authentication has been standing out as a non-trivial system that is essential for daily use in the manner of technological necessities. Although incredible developments have been reported in this field, current bio-authentication technologies indicate a number of issues arising in the means of security and reliability. Brain-Computer Interface (BCI) has been recently spotlighted to be suitable for biometric authentication and identification systems. Electroencephalography (EEG) based BCI technologies offer better resistance against spoof attempts among other conventional biometrics, such as fingerprints, retina, iris patterns, face or voice recognition. In this context, code-modulated visual evoked potentials (cVEPs) have spotlighted among several visual stimuli based BCI systems. In this study, we propose a cVEP-based Transaction Authentication Number (TAN) authenticator system integrated into a web browser and evaluate its feasibility and effectiveness for secure online transaction authentication. Our approach achieved an average accuracy of 88.53% across 17 participants.